The present invention relates generally to determining a kappa number of various wood pulps, and more particularly, relates to a system for rapidly determining the pulp kappa number of a pulp sample using a spectrophotometric detector.
The kappa number of pulp is an important parameter in pulp manufacturing. It is used for indirectly indicating lignin content, relative hardness, and bleachability of pulp. Once the kappa number is known, the amount of bleaching agent needed for achieving a desired pulp brightness can then be ascertained. The pulp kappa number can be determined by the volume of 0.02 mole/liter (0.1N) potassium permanganate (KMnO4) solution consumed through an oxidation reaction by 1 gram of moisture-free or oven dry (O.D.) pulp in an acidic medium, where potassium permanganate, also referred to herein simply as permanganate, is a strong oxidation agent.
Presently, the titration method is a universally known and commonly used method for measuring the kappa number of various pulps. This titration method is described in tappi Test Methodsxe2x80x94T236 cm-85, Tappi Press, 1996, which is incorporated herein by reference, and was first proposed in 1934 by Wiler, Paper Trade Journal, 98(11), 1934, and was later developed in the 1950s based on the work of Watson and Stamp, J. Aust. Pulp Paper Ind. Tech. Assoc., 11(1), 1957, Valeur and Torngren, Svensk Papperstidning, 30(22), 1957, and Tasman and Berzins, Tappi J., 40(9), 1957. Using the titration method, the pulp kappa number is calculated using the difference between the initial volume of potassium permanganate blank solution and the final volume of potassium permanganate remaining after the oxidation of lignin in the pulp-permanganate solution. It is known in the art that potassium permanganate blank solution is simply potassium permanganate solution without pulp.
The final volume is determined by titration to determine how much unconsumed potassium permanganate remains after a predetermined time period. In accordance with the method, the final volume is measured after ten minutes, thereby presuming that the oxidation of lignin in a fiber or pulp sample is complete after that time. Also, the titration method is performed wherein the pulp-permanganate solution temperature is maintained at 25xc2x0 C. and under an initial H+ concentration of 0.4 mol/L, or pH of about 0.4, acidic conditions,. The following reactions generally occur as a result of such acidic conditions:                                           MnO            4            -                    +                      8            ⁢                          H              +                                +                      5            ⁢            e                          ⁢                  →                                    E              0                        =                                          +                1.51                            ⁢              V                                      ⁢                              Mn                          2              +                                +                      4            ⁢                          H              2                        ⁢            O                                              (1a)                                                      MnO            4            -                    +                      4            ⁢                          H              +                                +                      3            ⁢            e                          ⁢                  →                                    E              0                        =                                          +                1.69                            ⁢              V                                      ⁢                              MnO            2                    +                      2            ⁢                          H              2                        ⁢            O                                              (1b)            
where (1a) produces manganese ions, Mn2+, and (1b) produces manganese dioxide precipitation, MnO2. Finally, the titration method requires that the consumed potassium permanganate volume is approximately 50% of the initial volume after a ten-minute reaction in order to obtain a valid measurement. Such a requirement is difficult to reconcile because the excess permanganate volume is known only after the kappa number is determined.
Under the titration method, a number of assumptions are made which can be problematic, and therefore, may lead to an inaccurate and inconsistent determination of the pulp kappa number. First, it is assumed that all of the lignin in pulp has reacted with the permanganate in the ten-minute time period. In fact, potassium permanganate can also be consumed by decomposition by other organic materials in pulp. Consequently, the final volume of potassium permanganate in the pulp-permanganate solution would not be a reflection of the excess potassium permanganate remaining after oxidation of lignin alone. Therefore, the difference between volumes calculated as the kappa number would be inaccurate.
Secondly, it is assumed that the oxidation reaction is completed in ten minutes. This oxidation reaction time is seemingly arbitrary in light of the fact that actual oxidation reaction times vary from pulp to pulp. Also, the oxidation reaction time is dependent on the mass of the pulp sample, thereby producing inconsistencies in kappa number.
Another assumption is that the effect of the variation in excess permanganate volume is insignificant. Due to its insignificance, the titration method attempts to correct this effect by a non-constant correction factor as tabulated in the Tappi test method. However, the non-constant correction factor is purely empirical through experimental calibration, and therefore, can lead to errors in determining the pulp kappa number.
Besides the problems that can arise due to the foregoing assumptions, the titration method itself is very tedious, time-consuming, and prone to error due to the human element. Titration is performed manually, and as a consequence, titration is very dependent upon the skill of an operator. It also takes about thirty minutes for an operator to complete the tedious titration process. For instance, pulp kappa numbers for the same pulp sample can also vary widely from one operator to the next operator.
Spectrophotometry is another technique that has been attempted to determine the pulp kappa number as described in articles entitled xe2x80x9cKappa Number Determination in Kraft Pulping by FTIR Spectroscopic Measurements on Spent Liquorsxe2x80x9d, Tappi J., 74(4):235 (1990), by A. J. Michell, xe2x80x9cDetermination of Total Lignin Content in Fibrous Materials,xe2x80x9d Zellstoff Papier, 23(11):327 (1974), by A. M. Plonka et al., and xe2x80x9cThe STFI OPTI-Kappa Analyzer: Applications and Accuracy,xe2x80x9d Tappi J., 70(11):38 (1987), by Kibulnieks et al., which are all incorporated herein by reference in their entireties. Spectrophotometry can provide direct and instantaneous measurements of chemicals in reactions by measuring the absorbance or transmittance of light from chemicals using an optical instrument commonly known as a spectrophotometer.
These methods described in the prior art are capable of ascertaining the lignin content of a pulp sample, but not the lignin reactivity, which can be used to determine the amount of chemicals required in the bleaching process. Consequently, these methods are limited in that they cannot directly ascertain the pulp kappa number, and therefore, use a pre-calibrated linear relationship between the lignin content in pulp and the pulp sample kappa number measured by the traditional titration method. The calibration relationship varies from pulp to pulp due to the variation of lignin reactivity with wood species.
Li and Gellerstedt, Nordic Pulp and Paper Research J., 13(2):147, 1998, reported the results of a study on the kinetics and mechanism of pulp-permanganate oxidation reaction under the conditions suggested in standard kappa testing methods. In the study, the permanganate in pulp-permanganate solutions was directly measured using UV/Vis spectrophotometry under conditions suggested in common kappa number test methods, such as TAPPI Test Methodxe2x80x94T236cm-85 and SCAN-C1 Test Method. According to Li and Gellerstedt, the dominant reaction is an overall conversion of permanganate to MnO2 precipitation according to reaction (1b). It was found that the precipitated MnO2 strongly interfered with the measured absorption spectrum of the pulp-reacting solution and caused significant measurement difficulties in determining the excess permanganate in the final pulp-reacting solution. It was impossible to obtain meaningful kappa numbers that agree with the kappa numbers obtained by standard titration based methods as they indicated in their work. Nevertheless, they concluded that direct spectrophotometry can be used to calculate the kappa number determination from their experimental data.
One problem with using their proposed method is since it is difficult to determine the amount of precipitated MnO2, the amount of permanganate consumed by pulp-permanganate oxidation reactions cannot be quantified for pulp kappa number calculations. In addition, considering the formation of MnO2 precipitation, one skilled in the art cannot use spectrophotometry to determine pulp kappa number due to suspended MnO2 particles producing spectral interference. Consequently, the spectral interference from the precipitated MnO2 typically causes errors in determining the pulp kappa number using spectrophotometry. This result cannot be ignored or eliminated. Hence, the Li and Gellerstedt conclusion cannot be supported by the reaction conditions described in the study, and the study does not describe or suggest how the kappa number may be directly measured using spectrophotometry.
Therefore, there is a need for an improved system and method for determining the pulp kappa number of a pulp sample. There is also a need for a system and method that determines the pulp kappa number accurately and consistently. In addition, there is a need for a system and method for determining the pulp kappa number for a pulp sample which is not operator dependent. There is a further need for a system and method for rapidly determining the pulp kappa number through direct measurements of a pulp sample. There is yet another need for a system and method for directly determining the pulp kappa number without the need for calibration or correction.
The present invention solves the above-described needs by providing a system and method for rapidly determining the pulp kappa number through direct measurement of the potassium permanganate concentration in a pulp-permanganate solution using spectrophotometry. Specifically, the present invention uses strong acidification to carry out the pulp-permanganate oxidation reaction in the pulp-permanganate solution to prevent the precipitation of manganese dioxide (MnO2). Consequently, spectral interference from the precipitated MnO2 is eliminated and the oxidation reaction (1a) becomes dominant. The spectral intensity of the oxidation reaction, also referred to herein as pulp-permanganate oxidation reaction, is then analyzed to determine the pulp kappa number. The term xe2x80x9cpulp-permanganate solutionxe2x80x9d is also referred to herein as pulp-potassium permanganate solution.
In addition, the present invention provides a method of accurately determining the pulp-permanganate oxidation reaction time and the final excess volume of potassium permanganate (KMnO4) in the pulp-permanganate solution from the measured permanganate absorption spectral intensities. Furthermore, due to the use of strong acidic conditions of the potassium permanganate solution, where the initial H+ concentration preferably greater than 3 mol/L, the present invention eliminates the effect of the final volume of excess potassium permanganate and the mass of the pulp, or pulp sample, on accurately determining the kappa number. Advantageously, the present invention then can be used to rapidly and directly determine pulp kappa numbers of various pulp samples using permanganate absorption data without the need for calibration and corrections. Finally, the present invention eliminates the element of human error common to prior methods for measuring kappa number.
One aspect of the present invention provides a method for determining a pulp kappa number for a pulp sample, comprising the steps of: mixing continuously a pulp-reacting solution, said pulp-reacting solution comprising: 1) a predetermined amount of pulp containing fibers and 2) a predetermined amount of reacting solution, said reacting solution having strong acidification and including an oxidation agent, where mixing the pulp-reacting solution causes an oxidation reaction in the pulp-reacting solution; filtering the pulp-reacting solution to limit fiber flow; subjecting at least a segment of the fiber filtered pulp-reacting solution to light, wherein the light is partially absorbed by the oxidation agent in the pulp-reacting solution; generating a time-dependent absorption spectral intensity from the absorption of the light by the oxidation agent in the pulp-reacting solution; collecting the time-dependent absorption spectral intensity by a absorption spectrographic method to determine an absorption spectral intensity for the oxidation agent; analyzing the time-dependent absorption spectral intensity using an oxidation reaction kinetic analysis to determine an oxidation reaction end point; and calculating a pulp kappa number for the pulp sample based on the predetermined amount of pulp, the predetermined amount of reacting solution, the absorption spectral intensity of the oxidation agent before the oxidation reaction, and the absorption spectral intensity of the oxidation agent at the oxidation reaction end point.
In accordance with this aspect of the present invention, the pulp-reacting solution may be subjected to light by an optical flow cell or a fiber optic probe, each being capable of transmitting and/or redirecting visible light. The predetermined amount of pulp ranges from about 0.05 to 1000 grams depending on the size of the reacting solution.
The reacting solution having strong acidification preferably contains sulfuric acid, where the sulfuric acid has a concentration level sufficient to prevent a significant amount of MnO2 from being precipitated during the oxidation reaction. Specifically, the reacting solution comprises the oxidation agent being 0.02 mol/L potassium permanganate solution and the strong acidification being 2.0 mol/L sulphuric acid, where the volume ratio of 0.02 mol/L potassium permanganate solution and 2.0 mol/L sulphuric acid is 1:4. Furthermore, the resulting sulfuric acid concentration in the pulp-reacting solution is preferably about 1.6 mol/L.
In addition, the strong acidification of the reacting solution can have an initial H+ concentration in the range of about 3.0 mol/L to 10 mol/L so long as it does not degrade the pulp fibers in the pulp-reacting solution.
The absorption spectroscopic method can use either the absorption spectral intensity of the oxidation agent at any selected wavelength between 450-600 nm or integrated over the entire absorption range of 450-600 nm.
After the spectral intensities have been obtained, the kappa number can be determined by calculating the kappa number based on the equation,       K    =                  a        w            ⁢              (                  1          -                                    A              e                                      A              0                                      )              ,
where K is the kappa number, a is the predetermined amount of reacting solution having strong acidification, w is the predetermined amount of pulp, A0 is the oxidation agent absorption before the oxidation reaction occurs, and Ae is the oxidation agent absorption spectral intensity at the oxidation reaction end point. A0 can be determined by obtaining the oxidation agent absorption in the reacting solution before mixing the pulp and the reacting solution together.
Both A0 and Ae can be the absorption spectral intensities at a single wavelength, for example at 546 nm, or integrated over the entire permanganate absorption spectrum (450-600 nm). In the case of using the integrated spectral intensities, a simple photo detector is sufficient to conduct kappa measurement using the present invention instead of using a spectrometer.
In yet another aspect of the present invention, a system is provided for determining permanganate absorption spectral intensities in a pulp-permanganate solution, comprising: a spectrometer for separating and detecting absorption spectral data of the pulp-potassium permanganate solution based on wavelengths; a light transmitting device connected to the spectrometer, wherein the light transmitting device provides absorption data of the pulp-permanganate solution to the spectrometer, said absorption data being between 450 and 600 nm; a filter for filtering the pulp-potassium permanganate solution before the light transmitting device provides the absorption data of the pulp-permanganate solution to the spectrometer, a light source connected to the light transmitting device, wherein the light source provides light to the pulp-potassium permanganate solution for absorption; and a computer system connected to the spectrometer, the computer system containing software program for collecting, displaying, and analyzing the absorption data of the pulp-potassium permanganate solution.
In connection with this aspect, the light transmitting device can be either a flow cell or a fiber optic probe for transmitting and/or redirecting visible light. In the case of using a flow cell, a pump is connected to the flow cell so as to provide flow of the pulp-potassium permanganate to the flow cell.